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Modern modes of sediment distribution and the anthropogenic heavy metal pollution record in northeastern Beibu Gulf, south China sea
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Modern modes of sediment distribution and the anthropogenic heavy metal pollution record in northeastern Beibu Gulf, south China sea Rong Wanga,b, Dong Xua,b,∗, Qian Gea,b a b
Key Laboratory of Submarine Geosciences, Ministry of Natural Resources, Hangzhou, 310012, China Second Institute of Oceanography, Ministry of Natural Resources, Hangzhou, 310012, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Surface sediment Sediment dynamics Heavy metal Beibu gulf
During the expedition 908 survey in 2007, 539 seafloor surface sediment samples and two cores were collected over a narrow sector of the northeastern Beibu Gulf, South China Sea. Currently, three dynamic sedimentological processes prevail in the study area: circulation-controlled sand deposition, mud deposition under weak sediment dynamics, and fluvial input. Core A233 from the circulation-controlled sand area, with a 60 cm mixed layer, provides evidence of dynamics disturbance since the mid-Holocene. To reconstruct the anthropogenic heavy metal pollution history, we selected core A146 from the stable mud sector influenced by fluvial input. The dating of core A146 was based on 210Pb activity analysis, showing an ~90-year historical record in the upper 40 cm. The heavy metal contamination results showed a generally low pollution level. Nonetheless, increased pollution has happened since the 1950s, especially after 1978 A.D., corresponding to the beginning of China's reform and opening up.
1. Introduction Environmental pollution caused by heavy metals began with the domestication of fire (Nriagu, 1996); however, the cumulative Pb deposition since the first Industrial Revolution (the 1770s) is 1 order of magnitude higher than the deposition documented for the GrecoRoman times for large-scale atmospheric pollution in the Northern Hemisphere (Candelone et al., 1995). Despite occurring as natural constituents in the crust of the earth, heavy metals are primarily emitted by anthropogenic sources (Hutton and Symon, 1986; Nriagu, 1989). Anthropogenic sources of heavy metals are closely related to industrial and agricultural production, including fertilizers, pesticides, former and present mining sites, foundries and smelters, combustion byproducts and traffic (Duruibe et al., 2007; Wuana and Okieimen, 2011). Due to its toxic nature, heavy metal contamination could generate serious environmental and health hazards (Jacob et al., 2018). Living organisms require varying amounts of heavy metals. Humans require iron, cobalt, copper, manganese, molybdenum, and zinc (Lane and Morel, 2000). However, all highly concentrated metals are toxic (Chronopoulos et al., 1997) and have been reported to affect cellular organelles and components (Wang and Shi, 2001), causing DNA damage that may lead to carcinogenesis (Wang and Shi, 2001; Beyersmann and Hartwig, 2008). Heavy metals can enter the human ∗
body system through food, air, water, and bioaccumulation (Lenntech, 2004). Because of the low solubility of the carbonate and hydroxide salts of the heavy metals in the ocean environment, heavy metal contaminants ultimately settle on the seabed in either soluble or particulate form (Ansari et al., 2004). As a major habitat and repository for heavy metal pollutants, sediments from the estuary and coastal areas could accumulate and record the discharge history of anthropogenic heavy metal contaminants (Loring, 1991; Audry et al., 2004; Xu et al., 2019). Contaminants are generally associated with fine sediments, which have a high sorptive capacity for problem contaminants (Ansari et al., 2004). The deposition of fine sediment is sensitive to the strength of transportation energy. However, the sedimentary dynamics are complicated and temperamental in both estuary and coastal areas. Proper study sites where stable terrigenous supplies can settle under weak sediment dynamics are essential. High-resolution surface sediment grain size can provide more evidence for distinguishing dynamic distributions in the study area. However, previous studies with limited (less than 100 surface samples) grain-size data from the eastern Beibu Gulf (Ma et al., 2010; Xu et al., 2010; Dou et al., 2012) could not offer details about the dynamic sedimentary boundaries in northeastern Beibu Gulf. The present study is based on seafloor surface sediment samples and two cores collected during the expedition 908 surveys in 2007 at nearly 539 representative sites, spanning the current highest resolution sector
Corresponding author. Key Laboratory of Submarine Geosciences, Ministry of Natural Resources, Hangzhou, 310012, China. E-mail address: [email protected] (D. Xu).
https://doi.org/10.1016/j.marpolbul.2019.110694 Received 17 July 2019; Received in revised form 17 October 2019; Accepted 25 October 2019 0025-326X/ © 2019 Published by Elsevier Ltd.
Please cite this article as: Rong Wang, Dong Xu and Qian Ge, Marine Pollution Bulletin, https://doi.org/10.1016/j.marpolbul.2019.110694
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Fig. 1. (a) Location and bathymetric map of the Beibu Gulf. The red and black rectangles mark the areas shown in panels (b) and (c), respectively; (b) Beibu Gulf. The black dots indicate study sites of seafloor surface sediments. The modern current system includes the winter (blue arrows) and summer (red arrows) circulations (Gao et al., 2017); (c) Northeastern Beibu Gulf. The green square and red pentagram indicate core locations in the study area: A146 (green square) and A233 (red pentagram). Drawn with Ocean Data View (Schlitzer, 2002). (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.) 2
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cores were collected during the expedition 908 surveys in 2007 at the locations shown in Fig. 1c. We analyzed the surface samples for grain size. Core A146, with a length of 132 cm and a water depth of 9 m, was obtained from a mud deposit south of Qinzhou Bay. Core A233, with a length of 154 cm and a water depth of 13 m, was obtained from a sand deposit off the western Beihai peninsula. The two cores were sampled in 2 cm slices for grain-size analysis. A total of 13 subsamples of core A146 at intervals of 5 cm (0–35 cm), 10 cm (35–65 cm) and 20 cm (65–125 cm) were analyzed for 210Pb activity. A total of 14 subsamples of core A233 at intervals of 5 cm (0–35 cm), 10 cm (35–65 cm) and 20 cm (65–145 cm) were analyzed for 210Pb activity. The subsamples of two cores at intervals of 5 cm (the upper 30 cm) and 10 cm (30 cmbottom) were selected for major and trace elemental analysis.
of the northeastern Beibu Gulf. This project seeks to (1) assess the statistical significance of terrigenous environmental signals documented in surface sediments, (2) identify potential study sites for future detailed paleoenvironmental reconstructions of heavy metal contaminant supplies and land-ocean linkages and (3) unravel continuous centennial heavy metal contaminant records in northeastern Beibu Gulf, South China Sea (SCS). 2. Materials and methods 2.1. Study area and regional setting As a semienclosed gulf, Beibu Gulf is located on the shelf of the northwestern SCS and is defined in the north, east and west by Guangxi Province, China, Leizhou Peninsula & Hainan Island, China, and the northern coastline of Vietnam, respectively (Fig. 1). The gulf seabed is flat, with an area of approximately 4.42 × 104 km2 and a water depth of 20–90 m. The Asian winter monsoon and Asian summer monsoon influence the Beibu Gulf alternately. In particular, the winter monsoon blows from northeast to southwest in the gulf at a speed of 9 m/s (Manh and Yanagi, 2000), showing a greater contribution to the circulation than the summer monsoon. Diurnal and semidiurnal tides from the Pacific Ocean through the Luzon Strait are dominant in the SCS. As a subarea of the SCS, the tidal energy flux in Beibu Gulf is essentially maintained by diurnal tides. Tidal waves are driven northeast towards the northern Beibu Gulf and are partially reflected, causing resonation of principal lunar diurnal components O1 (period: 25.82 h) and K1 (period: 23.93 h) and the highest increased amplitudes from the mouth to the head in the SCS (Fang et al., 1999; Minh et al., 2014). The wind, density gradient, and westward current from the Qiongzhou Strait and tidal rectification are major driving factors of the circulations. The northern and southern gyres compose the surface general circulation in the Beibu Gulf. In the northern gulf, the water circulation is mainly influenced by wind stress curl and dominated by the cyclones in both summer and winter. In the southern gulf influenced by the SCS current, the dominant water circulations are anticyclonic and cyclonic gyres in summer and winter, respectively. Despite the controversy, the current in the Qiongzhou Strait is conventionally considered to be eastward in summer and westward in winter (Gao et al., 2017; Xu et al., 2019) (Fig. 1b). As the largest river flowing into the gulf, the Red River carries approximately 40 × 106 t of sediment flux annually (Le et al., 2007). Some small rivers, e.g., Nanliu River, Qinjiang River, Jiuzhou River, and Beilun River, contribute limited sediment loads from the northern terrigenous area (Zhao et al., 2002; Li et al., 2017). In addition to fluvial runoff, the northeastern gulf is influenced by sediment supply from the ocean current through the southern mouth area from the SCS and through Qiongzhou Strait from the Pearl River (Zhou et al., 2014). According to the mineral analysis, erosion of the coast is another important sediment provenance in the northeastern Beibu Gulf (Chen and Zhang, 1986). The tidal ridges are mostly found around the western side of Hainan Island, while the estuary bars are towards the sea from sediment distribution along the eastern Beibu Gulf (Zhao et al., 2002). After the last transgression, a mud belt developed with a finer surface grain size (mean grain size less than 16 μm) outside of the surrounding sandy areas (Xu, 2014; Xu et al., 2019). Remarkable heavy metal contamination in sediments spread to the northern Beibu Gulf, e.g., the oil and gas exploration area (Xu et al., 2019), Nanliu River estuary (Xia et al., 2011), northern coast zones (Gan et al., 2013) and Red River estuary (Tue et al., 2012).
2.3. Analysis method Subsamples for grain-size analysis were pretreated with H2O2 (30%) and HCl (10%) to remove organic material and biogenic carbonate. Then, 100 ml of 0.05 M (NaPO3)6 was added to facilitate dispersion. Subsequently, the samples were rinsed with deionized water and homogenized by ultrasonic agitation for 30 s. Grain-size measurements were achieved by laser diffraction particle size analysis using a Malvern Mastersizer MAM 5005 at the Second Institute of Oceanography, Ministry of Natural Resources, China (SIO), following the method described in Asikainen et al. (2006) and providing data on 50 subclasses between 0.173 and 2000 μm. Since the data do not reveal simple unimodal distribution patterns, an endmember statistical analysis by Dietze et al. (2012) and Wang et al. (2016) is used to distinguish process-related sedimentary processes such as fluvial input, erosion and current transportation. For three samples of organic materials at depth ranges of 48–50, 98–00, and 148–150 cm from core A233, accelerator mass spectrometry (AMS) measurements of 14C were performed at Peking University, China. The age determinations followed established conventions (Stuiver and Polach, 1977). All corrected radiocarbon dates were converted to calibrated calendar ages (cal. yr BP) with the software Calib 7.0 (Stuiver and Reimer, 1993) and the INTCAL13 calibration curve (Reimer et al., 2013). All calibrated ages are weighted mean values with two sigma probabilities. For the 210Pb activity analysis performed at SIO, after freeze-drying and grinding, each 5 g sediment sample was dissolved by adding a mixture of HNO3 and HClO4 and heating to 90–95 °C. Additionally, 50 ml of 6 N hydrochloric acid and 1.0 ml trace agent 208Po standard were added into the flask. Then, the centrifugalized supernatant liquid was transferred to a settling bottle with ascorbic acid inside. A silver plate hung by nylon thread was soaked for 12 h in the liquid agitated by a magnetic stirrer for coating. Then, the Po was purified and self-plated onto the silver plate. 210Po activities in the coated silver plates were determined by high-resolution BH1324D multichannel α spectrometry with gold-silicon surface barrier detectors. Due to the secular equilibrium of the same activity between 210Pb and its granddaughter 210Po, we could calculate the 210Pb activity in the sediment (Robbins and Edgington, 1975; Robbins, 1978; Xu et al., 2019). A constant initial concentration (CIC) model has been applied to date the sediment core (Shukla and Joshi, 1989; De Souza et al., 2012). The whole sample elemental analysis was conducted in the Key Laboratory of Geochemical Exploration, Ministry of Land and Resources, China, following the methods described in Liu and Guang (1998) and Liang and Grégoire (2000) for both analysis and quality control. We determined the major elements in the sediment, such as Al2O3 and Fe2O3, using powder pellets and X-ray fluorescence spectrographic methods. The bulk sediments were digested by adding a mixture of HNO3, HClO4, and HF in a Teflon vessel. Trace elements (Cu, Zn, Pb, As, Hg, Cd, and Ag) were analyzed by using inductively coupled plasma mass spectrometry (ICP-MS, Thermo Fisher Scientific Inc., X Series II). Standard reference materials GBW07309 (GSD-9, Institute of
2.2. Sampling The study area extends across the northeastern realm of the Beibu Gulf (Fig. 1). In total, 539 surface samples (0–5 cm) and two gravity 3
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of the variance, respectively. The endmembers have their peak modes at 5–7 μm (EM1, fine silt; Figs. 2d), 700-900 μm (EM2, coarse sand; Figs. 2d) and 150–250 μm (EM3, fine sand; Fig. 2d). EM1 has a secondary peak of subordinate importance, probably due to statistically imperfect unmixing. The spatial distributions of the endmember scores in the northern Beibu Gulf (Fig. 2a–c) reveal that EM1 is dominant along the southern and western parts of the study area, tonguing northeast to the seashore, with top scores of up to 100%. EM2 has a negative correlation with EM1, showing high values and clear extension from northeast to southwest at the center of the study area. High to intermediate scores of EM3 are rare and confined to shallow sites along the estuary and coastline regions. The sediment provenance, sediment transport regimes, and current strength usually influence the nature of sedimentary grain-size endmembers (Wang et al., 2015, 2016). The statistical analysis of the grainsize data exhibits three significant populations, which can plausibly be attributed to the sedimentary dynamics being disturbed by the cyclonic water circulation (Fang et al., 1999; Gao et al., 2017). Strong wind stress and tidal amplitude interaction resulted in the transportation of fine sediment, leaving a substantial coarse sand (EM2) sector in the study area (Figs. 2b and 3). The fine-grained (EM1) nature of the northeastern Beibu Gulf sediments has been recognized in former studies and interpreted as a mud belt developed outside of the sandy areas after the last transgression in Beibu Gulf (Xu, 2014; Ni et al., 2016; Xu et al., 2019). The fine sand population of EM3 is typical for fluvial sediments along the northern Beibu Gulf continental margins (Gan et al., 2013) and the estuary of Qinzhou Bay (Gu et al., 2015; Meng et al., 2016).
Geophysical and Geochemical Exploration, Chinese Academy of Geological Sciences) were used for quality control. Together with reagent blanks and replicates, the differences in the concentrations between the determined and certified values were less than 1% and 5% for major and trace elements, respectively. To estimate the contamination degree of anthropogenic heavy metals in sediments, we use the common approach of normalized enrichment factor (EF) to reduce the influence of the mud/sand ratio and calculate the nondimensional heavy metal concentrations above background values. As a conservative element that is not related to other heavy metals, Al is widely used as a sample reference metal (Ravichandran et al., 1995; Dickinson et al., 1996; Abrahim and Parker, 2008; Huang et al., 2014). In this study, the EFs of the heavy metals were calculated using the following formula: EF=(heavy metal/ Al)sample/(heavy metal/Al)baseline, where (heavy metal/Al)sample is the ratio of the heavy metals to Al in the samples, while (heavy metal/ Al)baseline us the average ratio of the heavy metals to Al in preindustrial “baseline” sediments. The published calcium carbonate (CaCO3) contents and the ratios between total organic carbon (TOC) and total nitrogen (TN) is from Xu et al. (2013). We use the conductance potential method for TOC. Total carbon (TC) and TN were analyzed by using an elemental analyzer (EA3000). Then we calculated CaCO3 contents by TOC and TC. 3. Results and discussion 3.1. Grain-size distributions and sedimentary dynamics Endmember analysis of detrital grain-size distributions (Fig. 2) yields an optimal model with three endmembers (EMs) that explain 95% of the data variance, where EM1 to EM3 explain 86%, 7% and 2%
Fig. 2. Spatial distribution of score values for grain-size endmembers in seafloor surface sediments of the present study: (a) EM1, (b) EM2, and (c) EM3. (d) Typical grain-size distributions in the different endmembers from 539 surface samples. 4
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Fig. 3. Compilation of disparate sediment distributions in the study area inferred from the grain-size results of this study and the evaluation of literature data (Xu, 2014; Xu et al., 2019). Grain arrow modes and directions of fluvial sediment transport to core site A146 are shown, and the colored patches show the ranges of different sedimentary dynamics.
A233 and A146 (Fig. 4) yielded an optimal model with three endmembers (EMs) that together explain 98% of the data variance. Individually, EM1, EM2, and EM3 explain 85%, 11%, and 2% of the variance, respectively. The generated endmember loadings show peak modes at 4–6 μm and 100 μm (EM1, fine clay and very fine sand), 80–100 μm (EM2, very fine sand) and 20 μm (EM3, coarse silt). EM1 and EM3 both have secondary peaks of subordinate importance, which we attribute to statistically imperfect unmixing. The three AMS 14C results indicate a sedimentary history of the last 10 cal kyr archived in sediment core A233. Due to the high sedimentary
3.2. Dynamic restrictions on heavy metal contaminant records The active cyclonic water circulation plays a dominant role in the study area, leading to a large northeast-southwest tongue-shaped coarse sand area (Fig. 3). The high dynamic sorting effect resuspended and transported fine material to adjacent areas, depositing coarse materials in the areas with strong sediment dynamics. According to the surface grain-size distribution, core A233 is located in the northern part of the strong sediment dynamics sector. Endmember analysis of detrital grain-size distributions in cores
Fig. 4. Loadings of grain-size endmember populations in samples from sediment core A146 and core A233 (gray lines), showing typical grain-size distributions in the different endmembers (colored lines). 5
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Fig. 5. Depth profiles of grain-size endmember scores [%] of EM1–3 (see Figs. 4), 210Pb activities, CaCO3, and TOC/TN from core A233. Calibrated AMS 14C ages for the three A233 radiocarbon samples are shown on the left with depth coordinates. Sea-level change (Lambeck et al., 2014) during 10–0 cal kyr BP. Gray bars indicate periods of marine facies with strong sediment dynamics, erosion and reworking at the core site. The dotted line marks the water depth (−13 m) of core site A233. The CaCO3 and TOC/TN data for core A233 are from Xu et al. (2013).
located in the estuary of Qinzhou Bay, separated from the area with strong sediment dynamics by a wide mud belt (Fig. 3). An obvious tongue-shaped area with a relatively high EM3 (fluvial input) value extends from Qinzhou Bay to core site A146 (Fig. 2c), showing the strong influence of terrigenous sediment from Qinzhou Bay to core site A146. In this study, the site of core A146 is indeed related to sectors influenced by continuous river discharge and disturbance by weak sediment dynamics; therefore, it is considered to be a competent site for recording centennial anthropogenic heavy metal sediment contamination.
dynamics at the core site A233, we could not build an exact date-depth model. The downcore distributions of the endmember scores and 210Pb activities of A233 are shown in Fig. 5. Endmember EM2 is dominant throughout the upper section (0–66 cm), with proportions varying from 0% to 70%. However, the bottom section from depths of 66–154 cm shows absolutely 0% EM2. The 210Pb activity proxy in core A233 is inverted through the whole core compared with natural decay (Turner and Delorme, 1996; Xia et al., 2011), showing low values in the upper part (0–66 cm) and high values in the bottom part (66–154 cm). We compared our data with published CaCO3 contents and the ratios between TOC and TN from Xu et al. (2013), which have obvious transitions after 66 cm depth. The inverted lower Pb activities of core A233 show the nonexistence of modern deposits. In the bottom part, we interpret the fine grain size, high TOC/TN and low CaCO3 contents to correspond to exposed continental facies at the core site (Van den Bergh et al., 2007; Ge et al., 2010; Xu et al., 2013). During the early Holocene, the global eustatic sea level was lower than the level today by −40 m (10 cal kyr BP) to −13 m (8 cal kyr BP) (Tanabe et al., 2003; Lambeck et al., 2014), leaving a well-exposed shelf area close to the site of core A233 (−13 m) (Fig. 5). The shift also established the modern vigorous current system at the study site (Xu et al., 2013), which is well documented by coarse materials, low TOC/TN and high CaCO3 contents in the upper part of core A233. The winnowing processes leave behind the residual sandy sediments in the northeastern Beibu Gulf. The absent modern sediments, erosion, resuspension, and low sediment rates around the study site make a continuous recording of anthropogenic heavy metal contamination impossible. The high-resolution surface samples provide more detail of the sandy area and the boundary with the mud belt. In turn, core A146 is
3.3. Centennial heavy metal pollution record off Qinzhou Bay The downcore distributions of 210Pb activities and 210Pb excess activities of sediment core A146 are illustrated in Fig. 6. The 210Pb activities ranged from 0.839 to 5.22 dpm/g, showing an obvious receding from the top to 45 cm depth and a smooth graph at the bottom part. Average 210Pb activities (1.034 dpm/g) from 45 to 125 cm were used as the background value. The 210Pb excess activities are the 210Pb activities minus the background values. Due to the mixing effect at the top, we trisected the mean sedimentation rate to 0.38 cm/a, 0.14 cm/a, and 0.59 cm/a from the depths of 9 cm–35 cm with the CIC model (Appleby and Oldfield, 1978; Appleby, 1998; Xu, 2014; Xu et al., 2019) (Fig. 6). Therefore, the top 35 cm may have a sedimentation history of approximately 87 years, and the top 25 cm may represent the accumulation of sedimentary particles from 1950 to 2007. The downcore distributions of the heavy metal EF and endmember scores of A146 are shown in Fig. 7. Stratigraphic variations in the metal heavy mineral proxies of core A146 show a significant distinction between the upper and lower sections. Compared to the generally 6
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24% in the upper section, contributing to increased fine endmember EM1. EM1 has high scores from 0 cm to 32 cm with a mean fraction of 81%, while EM2 accounts for more than 30% in the top part of 0–5 cm. In general, concentrations of heavy metals and finer particles content in the sediment (d < 7.8 μm) have a strong correlation at the estuarine area (Zonta et al., 1994; Singh et al., 1999). The high scores of fine endmember EM1 make excellent for anthropogenic heavy metal deposition. Based on the chronology of core A146, 1949 A.D. and 1978 A.D. have been marked with dotted lines at ~12 cm and ~26 cm, respectively. Notable increases in EF (heavy metal) values have been recorded since the 1950s. In particular, a more rapid EF values increase occurs with an obvious drop of fine endmember EM1 from the 1980s to approximately 2007 A.D. These two milestones correspond to the foundation of the People's Republic of China in 1949 A.D. and the reform and opening up of China since 1978 A.D., respectively. According to Xia et al. (2011) and Gan et al. (2013), the sediment record from Nanliu and Qin River estuary showed consistently increasing tread of excess heavy metal since the 1980s. The neighbor sediment record from mud belt south of Qinzhou Bay has enrichment of Pb since the 1860s (Xu et al., 2019). The gross domestic product (GDP) and population of Guangxi Province have grown by 267 and 1.45 times during 40 years since 1978 A.D., respectively. In spite of natural origin for Pb and Cr (Xia et al., 2012; Xu et al., 2019), anthropogenic heavy metal contamination triggered by local economic development is a major source in marine sediments. The coarser fluvial endmember EM3 shows higher heavy metal concentrations than finer ones from the 1980s to approximately 2007 A.D, which could be contributed to the demographic expansion and intensified soil erosion. All EF values were lower than 2.0 throughout the whole core, even though an increase is obvious since the 1950s, indicating that heavy metal pollution is not serious in the historical record (Abrahim and Parker, 2008).
Fig. 6. Depth profile of 210Pb activities (dots) and 210Pb excess activities (crosses) for core A146. The solid and dotted lines indicate the three calculated stages of sediment rates until 2007 A.D.
featureless distribution (approximately 1.0) throughout the bottom core, the values of EF(Hg–Al), EF(Cu–Al), EF(Pb–Al) and EF(Zn–Al) show two sharp increases since depths of 25 cm and 11 cm, reaching a level generally higher than 1.0, with the highest value of 1.7 (EF (Hg–Al) at a depth of 5 cm). Coarse endmember EM2 ranges from 0% to
Fig. 7. EF values of heavy metal (Hg, Cd, Cu, Cr, Pb, As, Zn) and grain-size endmember scores [%] of EM1–3 (see Fig. 4) profiles from core A146. The two dotted lines mark the calculated dates of ~1949 A.D. and ~1978 A.D. based on the sedimentation rate. 7
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4. Conclusions
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Comput. Geosci. 28, 1211–1218. Shukla, B., Joshi, S., 1989. An evaluation of the CIC model of 210 Pb dating of sediments. Environ. Geol. Water Sci. 14, 73–76. Singh, A.K., Hasnain, S., Banerjee, D., 1999. Grain size and geochemical partitioning of heavy metals in sediments of the Damodar River–a tributary of the lower Ganga, India. Environ. Geol. 39, 90–98. Stuiver, M., Polach, H.A., 1977. Discussion; reporting of C-14 data. Radiocarbon 19, 355–363. Stuiver, M., Reimer, P.J., 1993. Extended 14C data base and revised Calib 3. 0 14C age calibration program. Radiocarbon 35, 215–230. Tanabe, S., Hori, K., Saito, Y., Haruyama, S., Kitamura, A., 2003. Song Hong (Red River) delta evolution related to millennium-scale Holocene sea-level changes. Quat. Sci. Rev. 22, 2345–2361. Tue, N.T., Quy, T.D., Amano, A., Hamaoka, H., Tanabe, S., Nhuan, M.T., Omori, K., 2012. Historical profiles of trace element concentrations in mangrove sediments from the Ba Lat Estuary, Red River. 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(1) Due to the interaction of tides, wind, circulation and terrigenous supply, three modern depositional processes of sediment endmembers prevail in the northeastern Beibu Gulf: circulation-controlled coarse sand deposition, mud deposition under weak sediment dynamics, and fluvial deposition. (2) Core A233, located in the sand deposit, showed the destructive disturbance of sediment dynamics after the transgression. Core site A146 is located at the mouth of Qinzhou Bay, separated from the area of strong sediment dynamics by a wide clay belt, showing a continuous sediment record. (3) The anthropogenic heavy metal contaminant record showed a generally low level of contamination in the northeastern Beibu Gulf. Nonetheless, the contamination growth rate has increased since the 1950s, especially after 1978 A.D., corresponding to the Chinese economic boom after economic reform. Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgments This work was jointly supported by the Scientific Research Fund of the Second Institute of Oceanography, Ministry of Natural Resources (grant No. 17010261), National Natural Science Foundation of China (Grant Nos. 41506064, 41427803), the National Programme on Global Change and Air-Sea Interaction (Grant No. GASI-GEOGE-03), and the State Oceanic Administration, China (Grant No. 201105003). We thank all the investigators on the cruises in 2007 for their help with sampling. References Abrahim, G., Parker, R., 2008. Assessment of heavy metal enrichment factors and the degree of contamination in marine sediments from Tamaki Estuary, Auckland, New Zealand. Environ. Monit. Assess. 136, 227–238. Ansari, T., Marr, I., Tariq, N., 2004. Heavy metals in marine pollution perspective—a mini review. J. Appl. Sci. 4, 1–20. Appleby, P., 1998. Dating recent sediments by 210 Pb: problems and solutions. In: Proceedings of a Seminar, pp. 7–24 Helsinki, 2–3 April, 1997. STUK-A 145. Appleby, P., Oldfield, F., 1978. The calculation of lead-210 dates assuming a constant rate of supply of unsupported 210Pb to the sediment. Catena 5, 1–8. Asikainen, C.A., Francus, P., Brigham-Grette, J., 2006. Sedimentology, clay mineralogy and grain-size as indicators of 65 ka of climate change from El’gygytgyn Crater Lake, Northeastern Siberia. J. Paleolimnol. 37, 105–122. Audry, S., Schäfer, J., Blanc, G., Jouanneau, J.-M., 2004. Fifty-year sedimentary record of heavy metal pollution (Cd, Zn, Cu, Pb) in the Lot River reservoirs (France). Environ. Pollut. 132, 413–426. Beyersmann, D., Hartwig, A., 2008. Carcinogenic metal compounds: recent insight into molecular and cellular mechanisms. Arch. Toxicol. 82, 493. Candelone, J.P., Hong, S., Pellone, C., Boutron, C., 1995. Post‐Industrial Revolution changes in large‐scale atmospheric pollution of the northern hemisphere by heavy metals as documented in central Greenland snow and ice. J. Geophys. Res.: Atmosphere 100, 16605–16616. Chen, L., Zhang, X., 1986. Mineral assemblages and their distribution pattern of the sediments from the Beibu Gulf. Acta Oceanol. Sin. 8, 340–346. Chronopoulos, J., Haidouti, C., Chronopoulou-Sereli, A., Massas, I., 1997. Variations in plant and soil lead and cadmium content in urban parks in Athens, Greece. Sci. Total Environ. 196, 91–98. De Souza, V.L., Rodrigues, K.R., Pedroza, E.H., Melo, R.T.d., Lima, V.L.d., Hazin, C.A., de Almeida, M.G., Nascimento, R.K.d., 2012. Sedimentation rate and 210Pb sediment dating at apipucos reservoir, recife, Brazil. Sustainability 4, 2419–2429. Dickinson, W., Dunbar, G., McLeod, H., 1996. Heavy metal history from cores in Wellington Harbour, New Zealand. Environ. Geol. 27, 59–69. Dietze, E., Hartmann, K., Diekmann, B., IJmker, J., Lehmkuhl, F., Opitz, S., Stauch, G., Wünnemann, B., Borchers, A., 2012. An end-member algorithm for deciphering modern detrital processes from lake sediments of Lake Donggi Cona, NE Tibetan Plateau, China. Sediment. Geol. 243, 169–180. Dou, Y., Li, J., Li, Y.J.G., 2012. Rare earth element compositions and provenance implication of surface sediments in the eastern Beibu Gulf. Geochimica 41, 147–157 (In Chinese). Duruibe, J.O., Ogwuegbu, M., Egwurugwu, J., 2007. Heavy metal pollution and human
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China): the background levels and recent contamination. Environ. Earth Sci. 66, 1337–1344. Xu, D., 2014. Sedimentary Records since Last Deglaciation and the Formation of Modern Sedimentary Pattern in Eastern Beibu Gulf. Institute of Oceanology, Chinese Academy of Sciences (in Chinese). Xu, D., Li, J., Chu, F., Long, J., Ye, L., Han, X.B., 2013. The response of sedimentary records in eastern Beibu gulf to the last postglacial transgression and circulation. Earth Sci. 38, 70–82 (In Chinese). Xu, D., Wang, R., Wang, W., Ge, Q., Zhang, W., Chen, L., Chu, F., 2019. Tracing the source of Pb using stable Pb isotope ratios in sediments of eastern Beibu Gulf, South China Sea. Mar. Pollut. Bull. 141, 127–136. Xu, Z., Wang, Y., Li, Y., Ma, F., Zhang, F., Ye, C., 2010. Sediment transport patterns in the eastern Beibu Gulf based on grain-size multivariate statistics and provenance analysis. Acta Oceanol. Sin. 32, 67–78 (In Chinese). Zhao, J.S., Wang, G.Y., Chen, Z.S., 2002. A Research on the Efficient Application of the Marine Environment: On Beibu Gulf and the Seacoast of Guangxi. China Ocean Press, Beijing. Zhou, S., Liu, Z., Zhao, Y., Stattegger, K., Wiesner, M., 2014. A high-resolution clay mineralogical record and its paleoenvironmental significance in the northeastern Gulf of Tonkin over the past 2000 years. Quat. Sci. 34, 600–610. Zonta, R., Zaggia, L., Argese, E., 1994. Heavy metal and grain-size distributions in estuarine shallow water sediments of the Cona Marsh (Venice Lagoon, Italy). Sci. Total Environ. 151, 19–28.
Turner, L., Delorme, L., 1996. Assessment of 210 Pb data from Canadian lakes using the CIC and CRS models. Environ. Geol. 28, 78–87. Van den Bergh, G., van Weering, T.C., Boels, J., Duc, D., Nhuan, M., 2007. Acoustical facies analysis at the Ba lat delta front (Red River delta, north Vietnam). J. Asian Earth Sci. 29, 532–544. Wang, R., Biskaborn, B.K., Ramisch, A., Ren, J., Zhang, Y., Gersonde, R., Diekmann, B., 2016. Modern modes of provenance and dispersal of terrigenous sediments in the North Pacific and Bering Sea: implications and perspectives for palaeoenvironmental reconstructions. Geo Mar. Lett. 36, 259–270. Wang, R., Zhang, Y., Wünnemann, B., Biskaborn, B.K., Yin, H., Xia, F., Zhou, L., Diekmann, B., 2015. Linkages between Quaternary climate change and sedimentary processes in Hala Lake, northern Tibetan Plateau, China. J. Asian Earth Sci. 107, 140–150. Wang, S., Shi, X., 2001. Molecular mechanisms of metal toxicity and carcinogenesis. Mol. Cell. Biochem. 222, 3–9. Wuana, R.A., Okieimen, F.E., 2011. Heavy metals in contaminated soils: a review of sources, chemistry, risks and best available strategies for remediation. Isrn Ecol. 1–20 2011. Xia, P., Meng, X., Yin, P., Cao, Z., Wang, X., 2011. Eighty-year sedimentary record of heavy metal inputs in the intertidal sediments from the Nanliu River estuary, Beibu Gulf of South China Sea. Environ. Pollut. 159, 92–99. Xia, P., Meng, X., Feng, A., Yin, P., Zhang, J., Wang, X.J.E.e.s., 2012. Geochemical characteristics of heavy metals in coastal sediments from the northern Beibu Gulf (SW
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Marine Pollution Bulletin journal homepage: www.elsevier.com/locate/marpolbul
Modern modes of sediment distribution and the anthropogenic heavy metal pollution record in northeastern Beibu Gulf, south China sea Rong Wanga,b, Dong Xua,b,∗, Qian Gea,b a b
Key Laboratory of Submarine Geosciences, Ministry of Natural Resources, Hangzhou, 310012, China Second Institute of Oceanography, Ministry of Natural Resources, Hangzhou, 310012, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Surface sediment Sediment dynamics Heavy metal Beibu gulf
During the expedition 908 survey in 2007, 539 seafloor surface sediment samples and two cores were collected over a narrow sector of the northeastern Beibu Gulf, South China Sea. Currently, three dynamic sedimentological processes prevail in the study area: circulation-controlled sand deposition, mud deposition under weak sediment dynamics, and fluvial input. Core A233 from the circulation-controlled sand area, with a 60 cm mixed layer, provides evidence of dynamics disturbance since the mid-Holocene. To reconstruct the anthropogenic heavy metal pollution history, we selected core A146 from the stable mud sector influenced by fluvial input. The dating of core A146 was based on 210Pb activity analysis, showing an ~90-year historical record in the upper 40 cm. The heavy metal contamination results showed a generally low pollution level. Nonetheless, increased pollution has happened since the 1950s, especially after 1978 A.D., corresponding to the beginning of China's reform and opening up.
1. Introduction Environmental pollution caused by heavy metals began with the domestication of fire (Nriagu, 1996); however, the cumulative Pb deposition since the first Industrial Revolution (the 1770s) is 1 order of magnitude higher than the deposition documented for the GrecoRoman times for large-scale atmospheric pollution in the Northern Hemisphere (Candelone et al., 1995). Despite occurring as natural constituents in the crust of the earth, heavy metals are primarily emitted by anthropogenic sources (Hutton and Symon, 1986; Nriagu, 1989). Anthropogenic sources of heavy metals are closely related to industrial and agricultural production, including fertilizers, pesticides, former and present mining sites, foundries and smelters, combustion byproducts and traffic (Duruibe et al., 2007; Wuana and Okieimen, 2011). Due to its toxic nature, heavy metal contamination could generate serious environmental and health hazards (Jacob et al., 2018). Living organisms require varying amounts of heavy metals. Humans require iron, cobalt, copper, manganese, molybdenum, and zinc (Lane and Morel, 2000). However, all highly concentrated metals are toxic (Chronopoulos et al., 1997) and have been reported to affect cellular organelles and components (Wang and Shi, 2001), causing DNA damage that may lead to carcinogenesis (Wang and Shi, 2001; Beyersmann and Hartwig, 2008). Heavy metals can enter the human ∗
body system through food, air, water, and bioaccumulation (Lenntech, 2004). Because of the low solubility of the carbonate and hydroxide salts of the heavy metals in the ocean environment, heavy metal contaminants ultimately settle on the seabed in either soluble or particulate form (Ansari et al., 2004). As a major habitat and repository for heavy metal pollutants, sediments from the estuary and coastal areas could accumulate and record the discharge history of anthropogenic heavy metal contaminants (Loring, 1991; Audry et al., 2004; Xu et al., 2019). Contaminants are generally associated with fine sediments, which have a high sorptive capacity for problem contaminants (Ansari et al., 2004). The deposition of fine sediment is sensitive to the strength of transportation energy. However, the sedimentary dynamics are complicated and temperamental in both estuary and coastal areas. Proper study sites where stable terrigenous supplies can settle under weak sediment dynamics are essential. High-resolution surface sediment grain size can provide more evidence for distinguishing dynamic distributions in the study area. However, previous studies with limited (less than 100 surface samples) grain-size data from the eastern Beibu Gulf (Ma et al., 2010; Xu et al., 2010; Dou et al., 2012) could not offer details about the dynamic sedimentary boundaries in northeastern Beibu Gulf. The present study is based on seafloor surface sediment samples and two cores collected during the expedition 908 surveys in 2007 at nearly 539 representative sites, spanning the current highest resolution sector
Corresponding author. Key Laboratory of Submarine Geosciences, Ministry of Natural Resources, Hangzhou, 310012, China. E-mail address: [email protected] (D. Xu).
https://doi.org/10.1016/j.marpolbul.2019.110694 Received 17 July 2019; Received in revised form 17 October 2019; Accepted 25 October 2019 0025-326X/ © 2019 Published by Elsevier Ltd.
Please cite this article as: Rong Wang, Dong Xu and Qian Ge, Marine Pollution Bulletin, https://doi.org/10.1016/j.marpolbul.2019.110694
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Fig. 1. (a) Location and bathymetric map of the Beibu Gulf. The red and black rectangles mark the areas shown in panels (b) and (c), respectively; (b) Beibu Gulf. The black dots indicate study sites of seafloor surface sediments. The modern current system includes the winter (blue arrows) and summer (red arrows) circulations (Gao et al., 2017); (c) Northeastern Beibu Gulf. The green square and red pentagram indicate core locations in the study area: A146 (green square) and A233 (red pentagram). Drawn with Ocean Data View (Schlitzer, 2002). (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.) 2
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cores were collected during the expedition 908 surveys in 2007 at the locations shown in Fig. 1c. We analyzed the surface samples for grain size. Core A146, with a length of 132 cm and a water depth of 9 m, was obtained from a mud deposit south of Qinzhou Bay. Core A233, with a length of 154 cm and a water depth of 13 m, was obtained from a sand deposit off the western Beihai peninsula. The two cores were sampled in 2 cm slices for grain-size analysis. A total of 13 subsamples of core A146 at intervals of 5 cm (0–35 cm), 10 cm (35–65 cm) and 20 cm (65–125 cm) were analyzed for 210Pb activity. A total of 14 subsamples of core A233 at intervals of 5 cm (0–35 cm), 10 cm (35–65 cm) and 20 cm (65–145 cm) were analyzed for 210Pb activity. The subsamples of two cores at intervals of 5 cm (the upper 30 cm) and 10 cm (30 cmbottom) were selected for major and trace elemental analysis.
of the northeastern Beibu Gulf. This project seeks to (1) assess the statistical significance of terrigenous environmental signals documented in surface sediments, (2) identify potential study sites for future detailed paleoenvironmental reconstructions of heavy metal contaminant supplies and land-ocean linkages and (3) unravel continuous centennial heavy metal contaminant records in northeastern Beibu Gulf, South China Sea (SCS). 2. Materials and methods 2.1. Study area and regional setting As a semienclosed gulf, Beibu Gulf is located on the shelf of the northwestern SCS and is defined in the north, east and west by Guangxi Province, China, Leizhou Peninsula & Hainan Island, China, and the northern coastline of Vietnam, respectively (Fig. 1). The gulf seabed is flat, with an area of approximately 4.42 × 104 km2 and a water depth of 20–90 m. The Asian winter monsoon and Asian summer monsoon influence the Beibu Gulf alternately. In particular, the winter monsoon blows from northeast to southwest in the gulf at a speed of 9 m/s (Manh and Yanagi, 2000), showing a greater contribution to the circulation than the summer monsoon. Diurnal and semidiurnal tides from the Pacific Ocean through the Luzon Strait are dominant in the SCS. As a subarea of the SCS, the tidal energy flux in Beibu Gulf is essentially maintained by diurnal tides. Tidal waves are driven northeast towards the northern Beibu Gulf and are partially reflected, causing resonation of principal lunar diurnal components O1 (period: 25.82 h) and K1 (period: 23.93 h) and the highest increased amplitudes from the mouth to the head in the SCS (Fang et al., 1999; Minh et al., 2014). The wind, density gradient, and westward current from the Qiongzhou Strait and tidal rectification are major driving factors of the circulations. The northern and southern gyres compose the surface general circulation in the Beibu Gulf. In the northern gulf, the water circulation is mainly influenced by wind stress curl and dominated by the cyclones in both summer and winter. In the southern gulf influenced by the SCS current, the dominant water circulations are anticyclonic and cyclonic gyres in summer and winter, respectively. Despite the controversy, the current in the Qiongzhou Strait is conventionally considered to be eastward in summer and westward in winter (Gao et al., 2017; Xu et al., 2019) (Fig. 1b). As the largest river flowing into the gulf, the Red River carries approximately 40 × 106 t of sediment flux annually (Le et al., 2007). Some small rivers, e.g., Nanliu River, Qinjiang River, Jiuzhou River, and Beilun River, contribute limited sediment loads from the northern terrigenous area (Zhao et al., 2002; Li et al., 2017). In addition to fluvial runoff, the northeastern gulf is influenced by sediment supply from the ocean current through the southern mouth area from the SCS and through Qiongzhou Strait from the Pearl River (Zhou et al., 2014). According to the mineral analysis, erosion of the coast is another important sediment provenance in the northeastern Beibu Gulf (Chen and Zhang, 1986). The tidal ridges are mostly found around the western side of Hainan Island, while the estuary bars are towards the sea from sediment distribution along the eastern Beibu Gulf (Zhao et al., 2002). After the last transgression, a mud belt developed with a finer surface grain size (mean grain size less than 16 μm) outside of the surrounding sandy areas (Xu, 2014; Xu et al., 2019). Remarkable heavy metal contamination in sediments spread to the northern Beibu Gulf, e.g., the oil and gas exploration area (Xu et al., 2019), Nanliu River estuary (Xia et al., 2011), northern coast zones (Gan et al., 2013) and Red River estuary (Tue et al., 2012).
2.3. Analysis method Subsamples for grain-size analysis were pretreated with H2O2 (30%) and HCl (10%) to remove organic material and biogenic carbonate. Then, 100 ml of 0.05 M (NaPO3)6 was added to facilitate dispersion. Subsequently, the samples were rinsed with deionized water and homogenized by ultrasonic agitation for 30 s. Grain-size measurements were achieved by laser diffraction particle size analysis using a Malvern Mastersizer MAM 5005 at the Second Institute of Oceanography, Ministry of Natural Resources, China (SIO), following the method described in Asikainen et al. (2006) and providing data on 50 subclasses between 0.173 and 2000 μm. Since the data do not reveal simple unimodal distribution patterns, an endmember statistical analysis by Dietze et al. (2012) and Wang et al. (2016) is used to distinguish process-related sedimentary processes such as fluvial input, erosion and current transportation. For three samples of organic materials at depth ranges of 48–50, 98–00, and 148–150 cm from core A233, accelerator mass spectrometry (AMS) measurements of 14C were performed at Peking University, China. The age determinations followed established conventions (Stuiver and Polach, 1977). All corrected radiocarbon dates were converted to calibrated calendar ages (cal. yr BP) with the software Calib 7.0 (Stuiver and Reimer, 1993) and the INTCAL13 calibration curve (Reimer et al., 2013). All calibrated ages are weighted mean values with two sigma probabilities. For the 210Pb activity analysis performed at SIO, after freeze-drying and grinding, each 5 g sediment sample was dissolved by adding a mixture of HNO3 and HClO4 and heating to 90–95 °C. Additionally, 50 ml of 6 N hydrochloric acid and 1.0 ml trace agent 208Po standard were added into the flask. Then, the centrifugalized supernatant liquid was transferred to a settling bottle with ascorbic acid inside. A silver plate hung by nylon thread was soaked for 12 h in the liquid agitated by a magnetic stirrer for coating. Then, the Po was purified and self-plated onto the silver plate. 210Po activities in the coated silver plates were determined by high-resolution BH1324D multichannel α spectrometry with gold-silicon surface barrier detectors. Due to the secular equilibrium of the same activity between 210Pb and its granddaughter 210Po, we could calculate the 210Pb activity in the sediment (Robbins and Edgington, 1975; Robbins, 1978; Xu et al., 2019). A constant initial concentration (CIC) model has been applied to date the sediment core (Shukla and Joshi, 1989; De Souza et al., 2012). The whole sample elemental analysis was conducted in the Key Laboratory of Geochemical Exploration, Ministry of Land and Resources, China, following the methods described in Liu and Guang (1998) and Liang and Grégoire (2000) for both analysis and quality control. We determined the major elements in the sediment, such as Al2O3 and Fe2O3, using powder pellets and X-ray fluorescence spectrographic methods. The bulk sediments were digested by adding a mixture of HNO3, HClO4, and HF in a Teflon vessel. Trace elements (Cu, Zn, Pb, As, Hg, Cd, and Ag) were analyzed by using inductively coupled plasma mass spectrometry (ICP-MS, Thermo Fisher Scientific Inc., X Series II). Standard reference materials GBW07309 (GSD-9, Institute of
2.2. Sampling The study area extends across the northeastern realm of the Beibu Gulf (Fig. 1). In total, 539 surface samples (0–5 cm) and two gravity 3
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of the variance, respectively. The endmembers have their peak modes at 5–7 μm (EM1, fine silt; Figs. 2d), 700-900 μm (EM2, coarse sand; Figs. 2d) and 150–250 μm (EM3, fine sand; Fig. 2d). EM1 has a secondary peak of subordinate importance, probably due to statistically imperfect unmixing. The spatial distributions of the endmember scores in the northern Beibu Gulf (Fig. 2a–c) reveal that EM1 is dominant along the southern and western parts of the study area, tonguing northeast to the seashore, with top scores of up to 100%. EM2 has a negative correlation with EM1, showing high values and clear extension from northeast to southwest at the center of the study area. High to intermediate scores of EM3 are rare and confined to shallow sites along the estuary and coastline regions. The sediment provenance, sediment transport regimes, and current strength usually influence the nature of sedimentary grain-size endmembers (Wang et al., 2015, 2016). The statistical analysis of the grainsize data exhibits three significant populations, which can plausibly be attributed to the sedimentary dynamics being disturbed by the cyclonic water circulation (Fang et al., 1999; Gao et al., 2017). Strong wind stress and tidal amplitude interaction resulted in the transportation of fine sediment, leaving a substantial coarse sand (EM2) sector in the study area (Figs. 2b and 3). The fine-grained (EM1) nature of the northeastern Beibu Gulf sediments has been recognized in former studies and interpreted as a mud belt developed outside of the sandy areas after the last transgression in Beibu Gulf (Xu, 2014; Ni et al., 2016; Xu et al., 2019). The fine sand population of EM3 is typical for fluvial sediments along the northern Beibu Gulf continental margins (Gan et al., 2013) and the estuary of Qinzhou Bay (Gu et al., 2015; Meng et al., 2016).
Geophysical and Geochemical Exploration, Chinese Academy of Geological Sciences) were used for quality control. Together with reagent blanks and replicates, the differences in the concentrations between the determined and certified values were less than 1% and 5% for major and trace elements, respectively. To estimate the contamination degree of anthropogenic heavy metals in sediments, we use the common approach of normalized enrichment factor (EF) to reduce the influence of the mud/sand ratio and calculate the nondimensional heavy metal concentrations above background values. As a conservative element that is not related to other heavy metals, Al is widely used as a sample reference metal (Ravichandran et al., 1995; Dickinson et al., 1996; Abrahim and Parker, 2008; Huang et al., 2014). In this study, the EFs of the heavy metals were calculated using the following formula: EF=(heavy metal/ Al)sample/(heavy metal/Al)baseline, where (heavy metal/Al)sample is the ratio of the heavy metals to Al in the samples, while (heavy metal/ Al)baseline us the average ratio of the heavy metals to Al in preindustrial “baseline” sediments. The published calcium carbonate (CaCO3) contents and the ratios between total organic carbon (TOC) and total nitrogen (TN) is from Xu et al. (2013). We use the conductance potential method for TOC. Total carbon (TC) and TN were analyzed by using an elemental analyzer (EA3000). Then we calculated CaCO3 contents by TOC and TC. 3. Results and discussion 3.1. Grain-size distributions and sedimentary dynamics Endmember analysis of detrital grain-size distributions (Fig. 2) yields an optimal model with three endmembers (EMs) that explain 95% of the data variance, where EM1 to EM3 explain 86%, 7% and 2%
Fig. 2. Spatial distribution of score values for grain-size endmembers in seafloor surface sediments of the present study: (a) EM1, (b) EM2, and (c) EM3. (d) Typical grain-size distributions in the different endmembers from 539 surface samples. 4
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Fig. 3. Compilation of disparate sediment distributions in the study area inferred from the grain-size results of this study and the evaluation of literature data (Xu, 2014; Xu et al., 2019). Grain arrow modes and directions of fluvial sediment transport to core site A146 are shown, and the colored patches show the ranges of different sedimentary dynamics.
A233 and A146 (Fig. 4) yielded an optimal model with three endmembers (EMs) that together explain 98% of the data variance. Individually, EM1, EM2, and EM3 explain 85%, 11%, and 2% of the variance, respectively. The generated endmember loadings show peak modes at 4–6 μm and 100 μm (EM1, fine clay and very fine sand), 80–100 μm (EM2, very fine sand) and 20 μm (EM3, coarse silt). EM1 and EM3 both have secondary peaks of subordinate importance, which we attribute to statistically imperfect unmixing. The three AMS 14C results indicate a sedimentary history of the last 10 cal kyr archived in sediment core A233. Due to the high sedimentary
3.2. Dynamic restrictions on heavy metal contaminant records The active cyclonic water circulation plays a dominant role in the study area, leading to a large northeast-southwest tongue-shaped coarse sand area (Fig. 3). The high dynamic sorting effect resuspended and transported fine material to adjacent areas, depositing coarse materials in the areas with strong sediment dynamics. According to the surface grain-size distribution, core A233 is located in the northern part of the strong sediment dynamics sector. Endmember analysis of detrital grain-size distributions in cores
Fig. 4. Loadings of grain-size endmember populations in samples from sediment core A146 and core A233 (gray lines), showing typical grain-size distributions in the different endmembers (colored lines). 5
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Fig. 5. Depth profiles of grain-size endmember scores [%] of EM1–3 (see Figs. 4), 210Pb activities, CaCO3, and TOC/TN from core A233. Calibrated AMS 14C ages for the three A233 radiocarbon samples are shown on the left with depth coordinates. Sea-level change (Lambeck et al., 2014) during 10–0 cal kyr BP. Gray bars indicate periods of marine facies with strong sediment dynamics, erosion and reworking at the core site. The dotted line marks the water depth (−13 m) of core site A233. The CaCO3 and TOC/TN data for core A233 are from Xu et al. (2013).
located in the estuary of Qinzhou Bay, separated from the area with strong sediment dynamics by a wide mud belt (Fig. 3). An obvious tongue-shaped area with a relatively high EM3 (fluvial input) value extends from Qinzhou Bay to core site A146 (Fig. 2c), showing the strong influence of terrigenous sediment from Qinzhou Bay to core site A146. In this study, the site of core A146 is indeed related to sectors influenced by continuous river discharge and disturbance by weak sediment dynamics; therefore, it is considered to be a competent site for recording centennial anthropogenic heavy metal sediment contamination.
dynamics at the core site A233, we could not build an exact date-depth model. The downcore distributions of the endmember scores and 210Pb activities of A233 are shown in Fig. 5. Endmember EM2 is dominant throughout the upper section (0–66 cm), with proportions varying from 0% to 70%. However, the bottom section from depths of 66–154 cm shows absolutely 0% EM2. The 210Pb activity proxy in core A233 is inverted through the whole core compared with natural decay (Turner and Delorme, 1996; Xia et al., 2011), showing low values in the upper part (0–66 cm) and high values in the bottom part (66–154 cm). We compared our data with published CaCO3 contents and the ratios between TOC and TN from Xu et al. (2013), which have obvious transitions after 66 cm depth. The inverted lower Pb activities of core A233 show the nonexistence of modern deposits. In the bottom part, we interpret the fine grain size, high TOC/TN and low CaCO3 contents to correspond to exposed continental facies at the core site (Van den Bergh et al., 2007; Ge et al., 2010; Xu et al., 2013). During the early Holocene, the global eustatic sea level was lower than the level today by −40 m (10 cal kyr BP) to −13 m (8 cal kyr BP) (Tanabe et al., 2003; Lambeck et al., 2014), leaving a well-exposed shelf area close to the site of core A233 (−13 m) (Fig. 5). The shift also established the modern vigorous current system at the study site (Xu et al., 2013), which is well documented by coarse materials, low TOC/TN and high CaCO3 contents in the upper part of core A233. The winnowing processes leave behind the residual sandy sediments in the northeastern Beibu Gulf. The absent modern sediments, erosion, resuspension, and low sediment rates around the study site make a continuous recording of anthropogenic heavy metal contamination impossible. The high-resolution surface samples provide more detail of the sandy area and the boundary with the mud belt. In turn, core A146 is
3.3. Centennial heavy metal pollution record off Qinzhou Bay The downcore distributions of 210Pb activities and 210Pb excess activities of sediment core A146 are illustrated in Fig. 6. The 210Pb activities ranged from 0.839 to 5.22 dpm/g, showing an obvious receding from the top to 45 cm depth and a smooth graph at the bottom part. Average 210Pb activities (1.034 dpm/g) from 45 to 125 cm were used as the background value. The 210Pb excess activities are the 210Pb activities minus the background values. Due to the mixing effect at the top, we trisected the mean sedimentation rate to 0.38 cm/a, 0.14 cm/a, and 0.59 cm/a from the depths of 9 cm–35 cm with the CIC model (Appleby and Oldfield, 1978; Appleby, 1998; Xu, 2014; Xu et al., 2019) (Fig. 6). Therefore, the top 35 cm may have a sedimentation history of approximately 87 years, and the top 25 cm may represent the accumulation of sedimentary particles from 1950 to 2007. The downcore distributions of the heavy metal EF and endmember scores of A146 are shown in Fig. 7. Stratigraphic variations in the metal heavy mineral proxies of core A146 show a significant distinction between the upper and lower sections. Compared to the generally 6
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24% in the upper section, contributing to increased fine endmember EM1. EM1 has high scores from 0 cm to 32 cm with a mean fraction of 81%, while EM2 accounts for more than 30% in the top part of 0–5 cm. In general, concentrations of heavy metals and finer particles content in the sediment (d < 7.8 μm) have a strong correlation at the estuarine area (Zonta et al., 1994; Singh et al., 1999). The high scores of fine endmember EM1 make excellent for anthropogenic heavy metal deposition. Based on the chronology of core A146, 1949 A.D. and 1978 A.D. have been marked with dotted lines at ~12 cm and ~26 cm, respectively. Notable increases in EF (heavy metal) values have been recorded since the 1950s. In particular, a more rapid EF values increase occurs with an obvious drop of fine endmember EM1 from the 1980s to approximately 2007 A.D. These two milestones correspond to the foundation of the People's Republic of China in 1949 A.D. and the reform and opening up of China since 1978 A.D., respectively. According to Xia et al. (2011) and Gan et al. (2013), the sediment record from Nanliu and Qin River estuary showed consistently increasing tread of excess heavy metal since the 1980s. The neighbor sediment record from mud belt south of Qinzhou Bay has enrichment of Pb since the 1860s (Xu et al., 2019). The gross domestic product (GDP) and population of Guangxi Province have grown by 267 and 1.45 times during 40 years since 1978 A.D., respectively. In spite of natural origin for Pb and Cr (Xia et al., 2012; Xu et al., 2019), anthropogenic heavy metal contamination triggered by local economic development is a major source in marine sediments. The coarser fluvial endmember EM3 shows higher heavy metal concentrations than finer ones from the 1980s to approximately 2007 A.D, which could be contributed to the demographic expansion and intensified soil erosion. All EF values were lower than 2.0 throughout the whole core, even though an increase is obvious since the 1950s, indicating that heavy metal pollution is not serious in the historical record (Abrahim and Parker, 2008).
Fig. 6. Depth profile of 210Pb activities (dots) and 210Pb excess activities (crosses) for core A146. The solid and dotted lines indicate the three calculated stages of sediment rates until 2007 A.D.
featureless distribution (approximately 1.0) throughout the bottom core, the values of EF(Hg–Al), EF(Cu–Al), EF(Pb–Al) and EF(Zn–Al) show two sharp increases since depths of 25 cm and 11 cm, reaching a level generally higher than 1.0, with the highest value of 1.7 (EF (Hg–Al) at a depth of 5 cm). Coarse endmember EM2 ranges from 0% to
Fig. 7. EF values of heavy metal (Hg, Cd, Cu, Cr, Pb, As, Zn) and grain-size endmember scores [%] of EM1–3 (see Fig. 4) profiles from core A146. The two dotted lines mark the calculated dates of ~1949 A.D. and ~1978 A.D. based on the sedimentation rate. 7
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(1) Due to the interaction of tides, wind, circulation and terrigenous supply, three modern depositional processes of sediment endmembers prevail in the northeastern Beibu Gulf: circulation-controlled coarse sand deposition, mud deposition under weak sediment dynamics, and fluvial deposition. (2) Core A233, located in the sand deposit, showed the destructive disturbance of sediment dynamics after the transgression. Core site A146 is located at the mouth of Qinzhou Bay, separated from the area of strong sediment dynamics by a wide clay belt, showing a continuous sediment record. (3) The anthropogenic heavy metal contaminant record showed a generally low level of contamination in the northeastern Beibu Gulf. Nonetheless, the contamination growth rate has increased since the 1950s, especially after 1978 A.D., corresponding to the Chinese economic boom after economic reform. Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgments This work was jointly supported by the Scientific Research Fund of the Second Institute of Oceanography, Ministry of Natural Resources (grant No. 17010261), National Natural Science Foundation of China (Grant Nos. 41506064, 41427803), the National Programme on Global Change and Air-Sea Interaction (Grant No. GASI-GEOGE-03), and the State Oceanic Administration, China (Grant No. 201105003). We thank all the investigators on the cruises in 2007 for their help with sampling. References Abrahim, G., Parker, R., 2008. Assessment of heavy metal enrichment factors and the degree of contamination in marine sediments from Tamaki Estuary, Auckland, New Zealand. Environ. Monit. Assess. 136, 227–238. Ansari, T., Marr, I., Tariq, N., 2004. Heavy metals in marine pollution perspective—a mini review. J. Appl. Sci. 4, 1–20. Appleby, P., 1998. Dating recent sediments by 210 Pb: problems and solutions. In: Proceedings of a Seminar, pp. 7–24 Helsinki, 2–3 April, 1997. STUK-A 145. Appleby, P., Oldfield, F., 1978. The calculation of lead-210 dates assuming a constant rate of supply of unsupported 210Pb to the sediment. Catena 5, 1–8. Asikainen, C.A., Francus, P., Brigham-Grette, J., 2006. Sedimentology, clay mineralogy and grain-size as indicators of 65 ka of climate change from El’gygytgyn Crater Lake, Northeastern Siberia. J. Paleolimnol. 37, 105–122. Audry, S., Schäfer, J., Blanc, G., Jouanneau, J.-M., 2004. Fifty-year sedimentary record of heavy metal pollution (Cd, Zn, Cu, Pb) in the Lot River reservoirs (France). Environ. Pollut. 132, 413–426. Beyersmann, D., Hartwig, A., 2008. Carcinogenic metal compounds: recent insight into molecular and cellular mechanisms. Arch. Toxicol. 82, 493. Candelone, J.P., Hong, S., Pellone, C., Boutron, C., 1995. Post‐Industrial Revolution changes in large‐scale atmospheric pollution of the northern hemisphere by heavy metals as documented in central Greenland snow and ice. J. Geophys. Res.: Atmosphere 100, 16605–16616. Chen, L., Zhang, X., 1986. Mineral assemblages and their distribution pattern of the sediments from the Beibu Gulf. Acta Oceanol. Sin. 8, 340–346. Chronopoulos, J., Haidouti, C., Chronopoulou-Sereli, A., Massas, I., 1997. Variations in plant and soil lead and cadmium content in urban parks in Athens, Greece. Sci. Total Environ. 196, 91–98. De Souza, V.L., Rodrigues, K.R., Pedroza, E.H., Melo, R.T.d., Lima, V.L.d., Hazin, C.A., de Almeida, M.G., Nascimento, R.K.d., 2012. Sedimentation rate and 210Pb sediment dating at apipucos reservoir, recife, Brazil. Sustainability 4, 2419–2429. Dickinson, W., Dunbar, G., McLeod, H., 1996. Heavy metal history from cores in Wellington Harbour, New Zealand. Environ. Geol. 27, 59–69. Dietze, E., Hartmann, K., Diekmann, B., IJmker, J., Lehmkuhl, F., Opitz, S., Stauch, G., Wünnemann, B., Borchers, A., 2012. An end-member algorithm for deciphering modern detrital processes from lake sediments of Lake Donggi Cona, NE Tibetan Plateau, China. Sediment. Geol. 243, 169–180. Dou, Y., Li, J., Li, Y.J.G., 2012. Rare earth element compositions and provenance implication of surface sediments in the eastern Beibu Gulf. Geochimica 41, 147–157 (In Chinese). Duruibe, J.O., Ogwuegbu, M., Egwurugwu, J., 2007. Heavy metal pollution and human
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